1. Field of the Invention
The present invention relates to a microphone which is to be connected to and operated along with a plurality of microphones, and a microphone apparatus including the microphones.
2. Related Background Art
Among electret condenser microphones, a known electret condenser microphone of a drain output type (two-wire system) yields an output from a drain of a FET (field-effect transistor) as an impedance converter (refer to Japanese Unexamined Patent Application Publication No. H8-33090, for example). Such a type of an electret condenser microphone is referred to as a “plug-in power system,” for example. FIG. 5 illustrates a typical electret condenser microphone of a plug-in power type.
In FIG. 5, a microphone unit 100 includes a microphone capsule 110, which has an electret member in either a diaphragm or a fixed electrode, and a FET 120 as an impedance converter. In the plug-in power system, a load resistor RL and a DC power source Vcc are connected in series to a drain D of the FET 120 such that a variable voltage across the load resistor RL is obtained from output terminals T1 and T2.
The FET 120 is of a bias built-in type having a diode and a high-value resistor which are connected between a gate G and a source S. A drain current is fixed to a drain current value (Idss) at a voltage of 0 between the gate G and the source S. Connecting a plurality of microphone units 100 in parallel to the load resistor RL synthesizes audio signals from the microphone units.
The plurality of microphone units 100 connected to the load resistor RL, however, cause the DC voltage to vary across the load resistor RL depending on the number of units. The DC voltage across the load resistor RL thus approaches the power voltage of the DC power source Vcc depending on the number of connected units. The voltage between the drain D and the source S of the FET 120 is then extremely reduced, preventing the FET 120 from operating.
To prevent a reduction in operating current of the impedance converter due to parallel connection of the microphone units, the resistance value of the load resistor RL is properly switched depending on the number of connected microphone units. Since the resistance value of the load resistor RL, however, should be switched during installation or use, it is practically cumbersome and inconvenient.
In order to address such a cumbersome and inconvenient circumstance, a method is known to use an output transformer instead of the load resistor RL. According to the method, the voltage between the drain D and the source S of the FET 120 is not reduced regardless of the number of connected microphone units (consumption current). Unfortunately, the DC current flowing to the output transformer generates DC magnetization that leads to a decline in performance of the transformer.
To address the circumstances above, a known microphone apparatus has an output transformer and a current mirror circuit in an output circuit and a plurality of microphones can be connected in parallel to the apparatus without special adjustment (refer to Japanese Unexamined Patent Application Publication No. 2006-197284, for example).
FIG. 4 illustrates a typical microphone apparatus as disclosed in Japanese Unexamined Patent Application Publication No. 2006-197284. In the microphone apparatus shown in FIG. 4, a plurality of microphone 100a to 100c are connected in parallel to a microphone apparatus main body 200. Each of the microphone 100a to 100c has the same configuration as the electret condenser microphone unit shown in FIG. 5. The microphone units 100a to 100c have microphone capsules 110a to 110c, respectively, each having an electret member in either a diaphragm or a fixed electrode; and FETs 120a to 120c, respectively, each serving as an impedance converter. The fixed electrode is connected to a gate G of each of the FETs 120a to 120c. The diaphragm is grounded along with a source S of each of the FETs 120a to 120c. 
The apparatus main body 200 has a DC power source Vcc, an output transformer 201 outputting audio signals and used as a load, and a current mirror circuit 204. A primary winding 202 of the output transformer 201 is split into a first winding 202a and a second winding 202b by a center tap 202c. Drains D of the FETs 120a to 120c of the microphone units 100a to 100c, respectively, are connected in parallel to the first winding 202a. The current mirror circuit 204 is connected to the second winding 202b. The center tap 202c is connected to the positive electrode of the DC power source Vcc. A secondary winding 203 of the output transformer 201 is connected to an audio output circuit (not shown in the drawing).
In the microphone apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2006-197284, the current mirror circuit 204 can generate an operating current which increases according to an increase in the number of connected microphone units. Furthermore, in the primary winding 202 of the output transformer 201, the current flows in directions opposite to each other in the first winding 202a and the second winding 202b as viewed from the center tap 202c, thus preventing saturation of the DC current. The microphone apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2006-197284, however, requires the output transformer and the current mirror circuit and has a high AC impedance of the FET drains, thus susceptible to external noise.